This project will continue to look at changes in immunologic reactivity of mice and mouse cells as a function of age. The ability of mouse B cells to undergo mitogenesis on exposure to anti-immunoglobulins (anti-Ig) is an age-related function, not appearing until 6-7 months of age. The role of Ig's on the surface will be explored. The ability of older mice to manifest contact sensitivity in vivo and in vitro will be quantitated.